The Act of Union that created the United Kingdom of Great Britain
Wales had already been absorbed by King Henry VIII in 1536, without objection.
During the course of the "War of Spanish Succession" a desired of union between the two kingdoms in Britannia was imminent. We recall that the Romans called Britannia what is today Great Britain.
For over one hundred years the English had proposed marriage to Scotland. Many will remember the struggles of the Scottish rebel William Wallace in the movie Braveheart starring and directed by Mel Gibson, which exposes the Scottish fight against English occupation of Edward I in the thirteenth century.
Navies and pirates of both kingdoms needed to navigate with one united flag. That is how it all started.
Evolution of the British flag:
Saint George flag representing England:
In 1194, Richard I of England introduced the cross of Saint George in red on a white background as the national flag of England.
Scotland is represented by the cross of Saint Andrew:
Scotland is represented by the cross of Saint Andrew: The Cross of Saint Andrew has two diagonal white lines on a dark blue background. The April 12, 1606, a royal decree declared that the British ships should carry both flags.
Finally, the 1707 Act of Union between England and Scotland merged the two crosses on a single flag to represent the United Kingdom of Great Britain:
First flag of the Union: 1707
The 1707 Act of Union brought together the kingdoms of England and Scotland with a single Parliament and a single flag, to form the United Kingdom of Great Britain.